This invention relates to a composite plant including a gas turbine and a steam turbine in combination, and more particularly to the control of a steam turbine by adjusting a quantity of steam to be fed to the steam turbine.
Hitherto, in a composite plant including, in combination, a steam turbine, a gas turbine, and a waste heat boiler producing the steam serving as an operating fluid for the steam turbine by using exhaust gas from the gas turbine as a heat source, the opening of a steam regulating valve for use in adjusting high pressure steam to be fed to the steam turbine has been controlled by resorting to a speed signal of the steam turbine, which has been detected by a governor equipped for the steam turbine. In such a method, however, if there arises fluctuation in pressure of the steam from the waste heat boiler, the change in r.p.m. of the steam turbine takes place after the flow rate of the steam produced has changed. Thus, there is a time lag of a considerable extent from the time when the change in steam pressure takes place until the regulating valve is conrolled by the governor, thus leading to unsatisfactory controlling for the steam turbine.
Where there are provided in the waste heat boiler two heat exchangers producing high pressure steam and low pressure steam, then high pressure steam produced in the heat exchanger of the high pressure side is introduced through a regulating valve or a steam valve into the high pressure portion of steam turbine, while low pressure steam produced in the heat exchanger on the low pressure side is introduced through an injection valve i.e. a steam valve into the low pressure portion of the steam turbine. The injection valve, likewise the regulating valve, effects a speed control with the aid of the governor, with its opening maintained in the fully opened position, during the running of the steam turbine. However, there frequently arises fluctuation in temperature of exhaust gas coming from the gas turbine into the waste heat boiler, due to variation in the load exerted on the gas turbine or due to variation in the ambient temperature in the composite plant. This leads to change in a steam condition, i.e. steam pressure, of the steam to be introduced into the low pressure portion of the steam turbine from the heat exchanger on the low pressure side of the waste heat boiler, resulting in the failure to control the running of the steam turbine. In other words, because the injection valve merely serves as an overspeed preventive valve when the steam turbine is in the normal running, there has been experienced that, depending upon change in pressure of the low pressure steam produced in the heat exchanger on the low pressure side in the waste heat boiler, pressure of the low pressure steam becomes higher than the pressure of the operating steam flowing from the high pressure portion of the steam turbine towards the low pressure portion thereof, and as a result, the steam streams within the steam turbine are reversed to cause the steam to flow from the low pressure portion to the high pressure portion in the steam turbine, leading to a serious trouble in the steam turbine.